issue: `zodResolver` disagrees with `z.infer` using transform APIs in Zod v4

[Garrett-R]

Summary

We get disagreements in typing for at least the following transform APIs:

• .default() (see below)
• .coerce (CodeSandbox)
• .preprocess() (CodeSandbox)

Version Number

5.2.0

Codesandbox/Expo snack

here

Steps to reproduce

Observe this code (also in Codesandbox ^):

import z from "zod";
import { zodResolver } from "@hookform/resolvers/zod";
import { useForm, UseFormRegister } from "react-hook-form";

const Foo = ({}) => {
  const MySchema = z.strictObject({
    numApples: z.number().nullable().default(null),
    numBananas: z.number().nullable(), // (no prob here)
  });
  type MySchemaType = z.infer<typeof MySchema>;

  const { register } = useForm({ resolver: zodResolver(MySchema) });
  const register2: UseFormRegister<MySchemaType> = register; // Type error!

  return <h1>Zod Form</h1>;
};

export default Foo;

Expected behaviour

No type errors.

But TS gives this error:

Type '{ numBananas: number | null; numApples?: number | null | undefined; }' is not assignable to type '{ numApples: number | null; numBananas: number | null; }'

I think z.infer is getting numApples type right (number | null) while zodResolver gets it wrong (number | null | undefined).

I'm on latest versions of react-hook-form, @hookform/resolvers, and zod.

Code of Conduct

• I agree to follow this project's Code of Conduct

[Scrumplex]

Some people have suggested using standardSchemaResolver instead, but that one doesn't infer the types at all for me using Zod 4.

[LoickBde]

Same with z.preprocess(...)

The use of standardSchemaResolver instead of zodResolver as suggested by @Scrumplex seems to fix the issue. I don't know if it's optimal though (fairly new on React hook form and Zod)

[Garrett-R]

Added z.preprocess() to description above.

But I'm not seeing that standardSchemaResolver fixes the issue, I'm seeing:

Type 'FieldValues' is not assignable to type '{ numApples: number; }'.

(CodeSandbox)